The present invention relates generally to measurement technology, and more specifically to measurement accomplished by ratiometric analog to digital converter technology.
There exists a great number and variety of applications where it is desired to measure a value, such as resistance, voltage, or current, with a fixed resolution. Analog to digital converters (A/D converters) are often used to measure an actual value, analog in form, and then to convert this measurement to a digital value. Digital voltmeters (DVMs), for instance, employ an A/D converter such as a dual-slope or ratiometric A/D converter in order to generate a very precise digital readout of a dc voltage applied to its input; the use of a standard dual-slope A/D converter provides the advantage of requiring only a minimal amount of calibration of the DVM. In addition to the standard dual-slope A/D converter shown in FIG. 1, other examples of integrating-type A/D converters include a single slope A/D converter, a dual-slope A/D converter having matched dual integrators, and a dual-slope A/D converter having sample-and-hold circuitry.
As mentioned above, a standard dual-slope A/D converter may be used to measure an unknown analog value, such as resistance, voltage, or current, and convert the measured analog value to a digital value. Typically, when measuring an analog value offset from a common ground reference, the unknown analog value is measured with a resolution that is higher than the required resolution and then an offset value is subtracted from the unknown analog value and the result rounded off to the desired resolution. Suppose that 8 bits of resolution is desired. The measurement must be made with a higher resolution than 8 bits, such as 9 bits or 10 bits of resolution, and the measurement result, after subtraction of an offset value from the unknown analog value, must be rounded off to 8 bits of resolution. For instance, if a value between 2 volts and 4 volts is to be measured with 8 bits of accuracy equal to 2.sup.8 or 256 bits, then 9 bits of resolution, 2.sup.9 or 512 bits, must be used to make the actual measurement. Furthermore, if a value between 4 volts and 6 volts is to be measured with 8 bits of accuracy, then 10 bits of resolution equal to 2.sup.10 or 1024 bits must be used to make the actual measurement. After the measurement has been made with a higher degree of resolution than the desired resolution and after subtraction of an offset value from the unknown analog value, then the result is rounded off to the desired resolution, in this case 8 bits resolution.
This type of conversion requires an A/D converter, such as a standard dual-slope A/C converter, with a resolution and accuracy higher than the required resolution and some logic to perform the subtraction. A schematic diagram of a standard dual-slope A/D converter 10 which may be used is shown in FIG. 1. Dual-slope A/D converter 10 is comprised of a first current source 12 having a current I1, a second current source 14 having a current 12, a pre-load value block 16, a counter 18, a logic block 20, an analog switch 22, an integrator 24 which has a capacitor 26, and a comparator 28. The accuracy of integrator 24 is a function of several factors such as the accuracy of capacitor 26, the accuracy of the timing source provided to the dual-slope A/D converter, and the accuracy of a reference value. Dual-slope A/D converter 10 provides the advantage of being inherently noise immune because of the input signal integration it provides.
When the analog switch 22 is connected to current source 12, a current proportional to V1 is applied to the integrator 24 for a fixed period of time t1 and the output wave form 30 of the integrator charges to a voltage equal to V0+.DELTA.V. If V0 is the resulting voltage on the output wave form 30 of the integrator after an auto zero cycle, then errors induced by offset of the comparator 28 and the integrator 24 can be calibrated out, as shown in the equations below. At the end of t1, current 12 proportional to V2 is applied to integrator 24 through switch 22 and then counter 18 is started which counts the number of cycles, measuring time t2, required for the output wave form 30 of the integrator 24 to drop below voltage V0. Referring to FIG. 2, a prior art wave form of output wave form 30 is shown at time t1 and time t2.
Equations of the standard dual-slope A/D converter of FIG. 1 which reflect the prior art wave form of FIG. 2 will now be developed. These equations are based upon the principal that:
__________________________________________________________________________ (1) I = C * dv/dt, where I is current, C is the capacitance of capacitor 26 of integrator 24, and dv/dt is the variation of voltage with respect to time. It follows from equation 1, that: (2) I1 = C * .DELTA. v1/t1 and (3) I2 = C * .DELTA. v2/t2, where t1 is the fixed time, t2 is the measured time, v1 is the unknown analog voltage, and v2 is the reference voltage. If .DELTA. vl = .DELTA. v2, then: (4) I1 * t1 = I2 * t2 and since I1 = v1/R1 and I2 = v2/R2, and assuming R1 = R2, then: (5) v1 * t1 = v2 * t2. We can define a constant k, since time t1 is a fixed time, that is equal to: (6) k = t1/v2, which yields: (7) t2 = k * v1 __________________________________________________________________________
While the above implementation of a standard dual-slope converter does serve to convert a raw analog measurement to a corresponding raw digital value, it does not produce a digital value that is offset with respect to a given measurement range. It is desirable to have a digital value after the conversion is complete that contains an offset, so that a digital value of 0 indicates not a value of 0 Ohms, 0 volts, or 0 Amps, but rather the minimum value of a given range of values. In the above example of measuring voltage between 2 volts and 4 volts, for example, it would be desirable to have a converted digital value of 0 which is representative of the bottom of the measurement range or 2 volts. This may be expressed in equation form where the desired conversion value is represented by: EQU k(X.sub.unknown -X.sub.offset)
where k is a constant, X.sub.unknown is the unknown analog value being measured, and X.sub.offset is the offset value.